The invention is concerned with improvements in or relating to stabilisation devices for drill motors, particular but not exclusively for use with downhole motors for operation in a bore.
Down-hole motors, which may be positive displacement motors, turbo-drills or any suitable motor arrangement for operation within a bore or other confined passage, are conventionally fitted with stabiliser devices to guide the motor body, shaft and drill bit in the bore. It will be understood that an un-stabilised arrangement working at operational speeds can partake of a whirling action producing a spiralling motion which can seriously reduce the drilling rate, the effect being particularly severe in the case of certain types of geological formations in which the bore is being formed.
The stabilisers conventionally used comprise a series of vanes machined with or secured to the motor body and usually the bit itself. The gaps between the vanes permits the passage of drilling fluid between the motor and the drilling area. The motor body supports a so-called bit box, a tool bit attachment-means which connects a replaceable bit to the motor shaft. In assembly the bit is secured to the box by the use of tongs which grip the box, necessitating that the latter has sufficient length to permit proper grip.
Conventionally, stabilisation of the bit may be achieved by the use of a long gauge bit, but this is not normally preferred and the bits are more usually provided with stabilising vanes similar to those on the motor body. Even in this case, the introduction of a vaned stabiliser on the bit necessarily increases the distance by which the bit projects beyond the motor bit box adversely affecting stress levels in the tool.
Moreover, in the use of downhole motors, it is conventional practice to control the progress of drilling on a path including portions having changes of direction, by using the so-called "orientate and rotate" technique. This is achieved by providing the motor body with an arcuate or inclined portion so that the body housing has a bend built in to it, or to provide an in-line housing with eccentric or off-set stabilisers. When it is required to drill the bore in a straight line, the drill string is rotated while the drill is operating. To continue drilling with a change of direction, the string is orientated and locked against rotation. When drilling is re-commenced the desired change of direction may then be achieved. To "steer" the motor in such a manner to obtain effective directional control and keep any reduction in drilling rate to a minimum, it is desirable to keep the distance by which the bit projects beyond the bit box as short as possible.
It will be appreciated that there is a conflict of requirements between what is advantageous for the reduction of whirling or spiralling and what is advantageous for directional control.